Imprint lithography is a technique in which a patterned layer such as a masking layer is formed on a semiconductor substrate by the deposition of a curable imprintable medium. The curable imprintable medium is subsequently patterned by imprinting the medium with a patterned stamp, after which the curable imprintable medium is solidified e.g. when exposed to light, e.g. UV-light to initiate the curing reaction in the medium. After the curing reaction is completed, the stamp is removed from the medium to leave a patterned layer on the semiconductor substrate.
This technique has recently drawn considerable attention because it can potentially provide a significant cost reduction over traditional lithography process steps. U.S. Pat. No. 7,354,698 discloses an example of such an imprinting method, in which the curing of the curable imprintable medium is initiated prior to the stamp imprinting step such that non-light transparent stamps can be used.
WO 2008/053418 discloses a method for forming a relief layer employing a stamp having a stamping surface including a template relief pattern in which no UV light is needed to solidify the curable imprintable medium. A solution comprising a silicon oxide compound is sandwiched between a substrate surface and the stamp surface and dried while sandwiched. The silicon oxide compound comprises a silicon atom chemically bound to at least three oxygen atoms, with, in case the silicon atom is bound to three oxygen atoms, the silicon being bound to a non-oxygen atom via a bond that is chemically inert in the relief layer forming reaction. Such silicon oxide compounds, i.e. alkoxysilanes, have the benefit that a dense cross-linked network is formed, giving the cured material excellent resist characteristics. Such compounds undergo a hydrolysis reaction followed by a water or alcohol condensation reaction in which a siloxane is formed. A more detailed description of this reaction process is disclosed in S. K Young, Sol-Gel Science for Ceramic Materials, Material Matters, 2006, Vol. 1 (3), page 8.
However, it has been found by the inventors of the present invention that curable solutions based on these types of silicon oxide compounds suffer from problems that prevent the wide applicability of these compounds in imprint lithography. For instance, the curable solution has a limited shelf-life; even at low temperatures, e.g. −20° C., some curing still takes place in the solution, as has been determined by NMR spectroscopy.
Moreover, when applying the solution onto a carrier such as a semiconductor substrate, differences in the reactivity of the solution cause problems in the reproducibility of the patterned layer formations, because the quality, i.e. curing state, of the curable imprintable medium when depositing the medium on the substrate has an impact of the characteristics of the subsequently formed patterned layer. The inventors of the present invention have found that it is difficult to consistently reproduce the relief layers formed using this solution.
This problem has been addressed in the prior art by dilution of the curable compound and acid catalyst in the solution to limit the unwanted spontaneous curing reaction. However, this has the drawback that the curing reaction on the substrate can take a considerable time to complete; this is demonstrated in FIG. 1, which shows scanning electron microscope (SEM) images of a grating formed by sol-gel imprint lithography after keeping the layer for 5 minutes in the mould (right pane), 10 minutes in the mould (middle pane) and 20 minutes in the mould (left pane) at room temperature. The images clearly demonstrate that in order to obtain a good imprint definition, the layer needs to be in the mould in excess of 10 minutes, which can result in excessive total processing times, i.e. a throughput of only a few wafers per hour, or worse in cases where a substrate layer is patterned in a stepwise manner, such as in a step and flash imprint lithography (SFIL) process.